Abstract

The Begonia flora of Southeast Asia comprises more than 540 species. This exceptional species diversity and the wide distribution of the genus in tropical rainforests offers the opportunity to address biogeographical questions and to investigate the processes which underlie modern patterns of biodiversity, but also poses major taxonomic challenges. Only few apomorphies characterising infrageneric taxa in this large genus have been identified and delimitation of Asian Begonia sections is highly problematic. A robust phylogenetic framework of Asian Begonia informing taxonomic monographs and facilitating biogeographical and evolutionary studies is currently lacking.
Maximum parsimony, maximum likelihood and Bayesian analyses of plastid (ndhA intron, ndhF-rpl32 spacer, rpl32-trnL spacer; 115 taxa) and nuclear ribosomal (ITS; 89 taxa) sequence data were used to reconstruct the phylogeny of Southeast Asian Begonia and to determine whether major Asian sections are monophyletic. Morphological characters which are crucial in current sectional circumscriptions were mapped on the phylogeny to determine their degree of homoplasy and to assess their suitability in infrageneric classifications. Relaxed molecular clock analyses of a Cucurbitales-Fagales datatset (cpDNA: matK gene, rbcL gene, trnL intron, trnL-F spacer; 92 taxa; five fossil calibrations) and a Begoniaceae datatset (cpDNA: ndhA intron, ndhF-rpl32 spacer, rpl32-trnL spacer; 110 taxa; two alternative secondary calibrations), as well as ancestral area reconstructions were employed to elucidate temporal and spatial diversification patterns in Asian Begonia.
The results indicate that Asian and Socotran Begonia species form a well supported clade. Most major Asian sections are not supported as monophyletic and the strong systematic emphasis placed on single, homoplasious characters such as undivided placenta lamellae (section Reichenheimia) and fleshy pericarps (section Sphenanthera), and the recognition of sections primarily based on a plesiomorphic fruit syndrome and the absence of characteristic features of other taxa (section Diploclinium) has resulted in the circumscription of several highly polyphyletic sections. Ovary and fruit characters have traditionally played a major role in sectional delimitation, however the high level of homoplasy associated with these has obscured systematic relationships in Asian Begonia. Gene trees derived from separate analyses of the plastid and nuclear ribosomal data show congruent support for several major clades, but there is hard incongruence within the clades comprising species of the species-rich sections Platycentrum s.l. (including section Sphenanthera) and Petermannia s.l. (including section Symbegonia), indicating that hybridization might have had a significant impact on the evolution of the genus.
The molecular divergence ages and the biogeographical analyses indicate an initial diversification of Asian Begonia on the Indian subcontinent and in continental Southeast Asia in the Middle Miocene, and subsequent colonization of Malesia by multiple lineages. The predominant directional trend of the reconstructed dispersals between continental Asia and Malesia and within Malesia is from west to east including four independent dispersal events from continental Southeast Asia and the Malesian Sunda Shelf region to Wallacea dating from the Late Miocene to the Pleistocene. Dispersal across the ancient deep water channels separating intervening islands of the Sunda Shelf and Wallacea and subsequent successful colonisation of Wallacean islands seem to have been infrequent events during this period. This suggests that the water bodies which have separated the Sunda Shelf region from Wallacea have been distinct, yet porous barriers to the predominantly anemochorous dispersal in Begonia. The inferred timing of dispersals from the Sunda Shelf region to Wallacea is generally concordant with hypotheses about the geological history of the region, which indicate that the period from the Late Miocene onwards offered opportunities for dispersal to Wallacea and across Wallacea to New Guinea as substantial land masses emerged in Sulawesi and New Guinea, and newly emergent volcanic islands along the Sunda Arc, the Banda Arc and the Halmahera Arc formed potential routes for dispersal by island hopping.
The results further suggest that Begonia section Petermannia (>270 spp.) originated in the Malesian Sunda Shelf region, and subsequently dispersed to Wallacea, New Guinea and the Philippines. Lineages within this section diversified rapidly since the Pliocene with diversification peaking in the Pleistocene. The timing of diversifications coincides with orogenesis on Sulawesi and New Guinea, as well as pronounced glacioeustatic sea-level and climate fluctuations. It can be hypothesised that a complex interplay of extrinsic and intrinsic factors including the presence and formation of suitable microhabitats by orogenesis, cyclical vicariance by frequent habitat fragmentations and amalgamations caused by sea-level and climate fluctuations, as well as only weakly developed mechanisms to maintain species cohesion in fragmented habitats in Begonia could have driven speciation in allopatry and could have resulted in the remarkable Begonia species diversity found in Southeast Asia today.